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A High Binary Fraction for the Most Massive Close-In Giant Planets and Brown Dwarf Desert Members

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A High Binary Fraction for the Most Massive Close-In Giant Planets and Brown Dwarf Desert Members MNRAS 000,1–30 (2019) Preprint 29 March 2019 Compiled using MNRAS LATEX style file v3.0 A high binary fraction for the most massive close-in giant planets and brown dwarf desert members C. Fontanive1;2?, K. Rice1;2, M. Bonavita1;2, E. Lopez3;4, K. Mužic´5 and B. Biller1;2 1SUPA, Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK 2Centre for Exoplanet Science, University of Edinburgh, Edinburgh EH9 3HJ, UK 3NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA 4GSFC Sellers Exoplanet Environments Collaboration, NASA GSFC, Greenbelt, MD 20771, USA 5CENTRA, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, P-1749-016 Lisboa, Portugal Accepted 2019 March 5. Received 2019 February 27; in original form 2019 January 17 ABSTRACT Stellar multiplicity is believed to influence planetary formation and evolution, although the precise nature and extent of this role remain ambiguous. We present a study aimed at testing the role of stellar multiplicity in the formation and/or evolution of the most massive, close- in planetary and substellar companions. Using past and new direct imaging observations, as well as the Gaia DR2 catalogue, we searched for wide binary companions to 38 stars hosting massive giant planets or brown dwarfs (M > 7 MJup) on orbits shorter than ∼1 AU. We report the discovery of a new component in the WASP-14 system, and present an independent con- firmation of a comoving companion to WASP-18. From a robust Bayesian statistical analysis, +13:2 we derived a binary fraction of 79:0−14:7% between 20−10,000 AU for our sample, twice as high as for field stars with a 3-σ significance. This binary frequency was found to be larger than for lower-mass planets on similar orbits, and we observed a marginally higher binary rate for inner companions with periods shorter than 10 days. These results demonstrate that stellar companions greatly influence the formation and/or evolution of these systems, sug- gesting that the role played by binary companions becomes more important for higher-mass planets, and that this trend may be enhanced for systems with tighter orbits. Our analysis also revealed a peak in binary separation at 250 AU, highlighting a shortfall of close binaries among our sample. This indicates that the mechanisms affecting planet and brown dwarf for- mation or evolution in binaries must operate from wide separations, although we found that the Kozai-Lidov mechanism is unlikely to be the dominant underlying process. We conclude that binarity plays a crucial role in the existence of very massive short-period giant planets and brown dwarf desert inhabitants, which are almost exclusively observed in multiple systems. Key words: planetary systems – planets and satellites: formation – binaries: visual – binaries: close – methods: observational – methods: statistical 1 INTRODUCTION the characteristics and demographics of planetary populations (e.g. arXiv:1903.02332v2 [astro-ph.EP] 28 Mar 2019 Desidera & Barbieri 2007; Eggenberger et al. 2007, 2011; Daem- In the search for analogues to the planets in our own Solar System, gen et al. 2009; Adams et al. 2012, 2013; Ginski et al. 2012). The exoplanet studies originally firmly excluded known binary systems, dominant results that emerged from these surveys were a strong despite the fact that about half of Solar-type stars are found in mul- deficit of binary companions within ∼50−100 AU for planet hosts tiple systems (Raghavan et al. 2010). Serendipitous discoveries and (Roell et al. 2012; Bergfors et al. 2013; Wang et al. 2014a,b; Kraus subsequent dedicated surveys later revealed that a significant frac- et al. 2016), and the idea that massive short-period planets appear to tion of exoplanets are actually found in binary star systems (e.g. be preferentially found in multiple-star systems (Zucker & Mazeh Patience et al. 2002; Desidera et al. 2004; Mugrauer et al. 2006; 2002; Eggenberger et al. 2004). Mugrauer & Neuhäuser 2009), mostly with binary separations of at least a few hundred AU. These findings led to numerous stud- These studies, however, focused primarily on systems in ies investigating how stellar binarity affects planet formation and which the planet had a mass less than ∼4 MJup. Theoretical calcula- tions (Kratter et al. 2010; Forgan & Rice 2011) and numerical sim- ulations (Stamatellos & Whitworth 2008; Stamatellos 2013; Hall ? E-mail: [email protected] et al. 2017) both suggest that planets that form via disc fragmen- c 2019 The Authors 2 C. Fontanive et al. tation in gravitationally unstable discs (Boss 1998) typically have jects within the brown dwarf mass regime, have yet to be studied in masses above ∼4 MJup. Therefore the planets in these existing stud- detail in the context of stellar multiplicity. Zucker & Mazeh(2002) ies probably formed via the standard core accretion scenario (CA; were the first to point out that massive (Mp > 4 MJup) short-period Pollack et al. 1996), rather than via gravitational instability (GI). planets tend to be predominantly found orbiting one component of When it comes to planets that formed via core accretion, bina- a multiple star system and possess distinctive characteristics com- rity on close separations is generally considered to have a negative pared to planets orbiting single stars (Eggenberger et al. 2004). influence (see Thebault & Haghighipour 2015 for a review of planet Such massive planetary and substellar companions are very formation in binaries and the issues introduced by the presence of challenging to form at small separations. Giant planet formation, a close binary companion). Theoretical studies have concluded that whether by CA or GI, is thought to occur preferentially in the rel- close stellar companions can hinder planet formation by stirring up atively cool outer regions of protoplanetary discs, from a few AU protoplanetary discs (e.g. Mayer et al. 2005), tidally truncating the for CA (Pollack et al. 1996), to several tens of AU for GI (Rafikov discs (e.g. Pichardo et al. 2005; Kraus et al. 2012), or leading to the 2005). Massive hot Jupiters are thus expected to have formed at ejection of planets (Kaib et al. 2013; Zuckerman 2014). More com- wide orbital separations from their host stars, where the lower tem- pact, truncated discs generally have just enough mass left to form a peratures in the protoplanetary disc allow for planet formation to low-mass Jovian planet (Jang-Condell et al. 2008), and planet for- proceed (Bell et al. 1997), or be born under very different condi- mation is then further complicated by the very short lifetime (.1 tions than currently encompassed by most planet formation mod- Myr) of these truncated discs (Kraus et al. 2012). els. Recently, Schlaufman(2018) found evidence for two distinct On the other hand, Batygin et al.(2011) and Rafikov(2013) populations of close-in giant planets, with a suggestion of a tran- predict that stellar companions should have little influence on plan- sition between CA and GI companions occurring at around ∼4−10 etesimal growth. It has also been proposed that the presence of an MJup. This is consistent with both semi-analytic (Kratter et al. 2010; outer companion could raise spiral arms in protoplanetary discs, Forgan & Rice 2011) and numerical simulations (Stamatellos & creating regions of high gas and particle densities, favourable for Whitworth 2008; Stamatellos 2013; Hall et al. 2017) which sug- planetesimal formation (Youdin & Goodman 2005) and pebble ac- gest that objects that form via GI have masses above ∼3−5 MJup, cretion (Johansen et al. 2007; Lambrechts & Johansen 2014). For and might suggest that these more massive close-in planets formed example, the spiral arm structures observed in the disc around HD by GI rather than via CA. 100453 (Wagner et al. 2015) may be due to perturbations from the In this work, we aim to constrain the multiplicity statistics of M-dwarf companion (Dong et al. 2016), located at 120 AU from hosts to the most massive giant planets (Mp > 7 MJup) and brown the primary and originally reported by Chen et al.(2006). Simi- dwarfs found within ∼1 AU, in order to investigate the role of bina- larly, the asymmetric disc of HD 141569 is attributed to the stellar rity in the formation or evolution of these systems. This will allow companions in this triple system (Augereau & Papaloizou 2004). us to assess if a wide binary companion could be responsible for In the “Friends of hot Jupiters” series of papers, Knutson et al. the observed orbital configurations of these objects, which are both (2014), Piskorz et al.(2015) and Ngo et al.(2015, 2016) found scarce and challenging to explain with current formation theories. a binary fraction 3 times higher for hosts to hot Jupiters (mostly Our investigation will provide an indication of whether the Kozai- up to Mp ∼ 4 MJup) than for field stars on separations of 50−2000 Lidov mechanism could play a role in the origin of the most mas- AU, and concluded that wide binary systems may either facilitate sive short-period gas giant planets and brown dwarfs. This study the formation of Jovian planets, or help the inward migration of will also help us determine if these massive companions are an planets formed at wider separations. extension of the population of lower-mass, CA giant planets, or It has also been suggested that binary companions could in- if they belong to a separate population formed through a distinct duce the inward migration of planets through secular interactions mechanism (i.e. GI on wide orbits, followed by inward migration; such as the Kozai-Lidov mechanism (Kozai 1962; Lidov 1962). In Nayakshin 2010; Rice et al. 2015). In particular, we will explore the this scenario, an outer companion with a large mutual inclination binary properties of hosts to members of the “brown dwarf desert” between the planetary and binary orbits can excite large periodic (Marcy & Butler 2000), depicting the significant deficit of brown oscillations of the eccentricity and inclination of the planet.
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